The x6EM/X2 expansion modules for the x610 series switches is a 10Gb cable expansion modules that installs in the back of the x610 where the stacking modules would normally go. You can either stack across the modules or make them normal 10Gb interfaces.
Since they use sfp+ to connect, you can actually stack units at different physical locations. They do differ in the fact that stacking cables are 12Gb and the x6EM/X2 are 10 Gb, but as stated before can be used as an additional (2) 10 Gb ports if you do not want to stack the x610. This brings the x610 to (4) total 10 Gb interfaces.
By default the switch will consider the x6EM/X2 as stacking ports. If you do not want them as stacking ports, just run the commands:
no stack 1 enable
You will need to reboot the x610.
An easy way to see if the router is using them as 10 Gb interfaces is to run the command:
show int brief
At the bottom there will be port 1.1.1 and port 1.1.2. If you see these interfaces then they are recognized as normal 10Gb interfaces. If those ports are not showing then the x610 is using them for stacking.